The invention concerns a process for the production of an impregnated, dimension-stable polyester cord, of at least 95 percent by weight polyethylene terephthalate having a limit viscosity of more than 0.70 dl/g, and having a titer of at least 1100.times.2 dtex, by means of a two-stage heat treatment including hot stretching and hot relaxation, whereby the stretching follows at a higher temperature than the relaxation, as well as a polyester cord produced according to this process.
Motor vehicle tires of typical construction are composed in their essential parts of a stability support in the form of cord, which is vulcanized into rubber. The external shape and, in particular, the tread, i.e., the contact surface, is determined by quite specifically composed rubber mixtures. In order for such tires to possess good operating characteristics and a long useful life, two significant conditions, among others, must be fulfilled: the tires must be very uniform in their external shape and, accordingly, remain dimension-stable indeed upon production as well as in later use and with fast driving of vehicles.
When dimension-stable products such as e.g., metal wires, are employed for stability supports, the conditions are somewhat more easily attained through precise tire construction. However, as soon as materials such as twine of high strength synthetic yarns is employed, the objective is attained only to an unsatisfactory extent, since either during or directly after the vulcanization, indeed depending upon the processing conditions, the rubber of the stability supports shrinks or lengthens, resulting in an inability to maintain the stability support under control.
Processes are known for the two-stage dipping and hot stretching of polyester yarns. For example, according to DE-AS 20 22 748, a polyester yarn as used impregnated for tires is stretched in a first stage at least eight percent at a temperature between 199.degree. C. and 254.degree. C., and relaxed at least seven percent in second stage at a temperature between 163.degree. and 204.degree. C. The resulting polyester cord displays at 160.degree. C. a heat shrinkage not less than 5.8 percent. Whether or not the strength characteristics of such a cord are improved, the relatively high heat shrinkage causes an always still unsatisfactory dimension-stability in the prepared pneumatic tires.
The use of of a polyester filament yarn based upon at least 85 mol-percent polyethylene terephthalate in pneumatic tires is known from DE-AS 27 47 690. According to the described process, the characteristics of the yarn employed for construction are improved from previously more than 10 percent shrinkage up to between 7.8 and 5.0 percent shrinkage when they are heated. Whether or not the employment of polyester filament yarn having a pre-thermal shrinkage of about 5 percent is sufficient for many requirements, it is frequently not suitable to meet the requirements of pneumatic tires of higher demand ratings.
Such polyester filaments which shrink more than 5 percent, be it that the starting material per se already has this very high shrinkage or be it that in the course of production of the tires, directly after the vulcanization, the hot tires are subjected to high pressure (i.e., so-called post-cure inflation), are still not dimension-stable. In the course of employment their length changes, which leads to unsmooth running of the tires, since this change in length is uncontrolled.
It is therefore an object according to the present invention to make available a process for the production of an improved dimension-stable polyester cord which displays a minimal shrinkage during the vulcanization and later, during employment.
It is a further object according to the present invention to provide a process for the economical production of such dimension-stable cords having low shrinkage, sufficient initial modulus and improved fatigue resistance.